Why would an engine have very loud knocks momentarily after each oil change?

One of my parents’s previous cars had over 400.000KM (back when cars were made to last) that always had these really loud knocks momentarily after each oil change. It goes away after a while of idling and will not come back until the next oil change.

Why would it do that in the first place, and why did it stop until the next oil change.

9 Answers

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  • Anonymous
    1 month ago

    The new oil filter is empty and until it fills up, the oil pressure is too low to prevent metal-to-metal friction. I suspect the connecting rod bearings were very worn out. It also could be valve lifters needed to pump up to close rocker lash and clatter. 

  • 1 month ago

    Because the oil pump first fills the *empty oil filter before providing oil pressure to the lubricated parts of the engine. An easy way to prevent the engine "rattle" after an oil change is to *fill the new oil filter to the top before screwing it back in the engine!! Believe me, it works every time! This is an old Bill "Grumpy" Jenkins (engine builder-drag racer trick that I've been doing 60 years.

  • Anonymous
    1 month ago

    Jan? Think 'bout it. The oil hasn't circulated yet.

  • M.
    Lv 7
    1 month ago

    Since I started maintaining and repairing automobiles in 1971, I always filled any vertically oriented (threaded end up) oil filter full with oil at the oil and oil filter change.  Horizontally or angularly oriented oil filters got some oil poured in. 

    If you start an engine with a new/empty oil filter, then it takes time to fill the oil filter with oil before the crankshaft bearings get pressure fed oil.  That is the knocking noise; connecting rod bearings with no oil being force fed to them.  It could take a few seconds or 5 seconds or maybe even longer. 

    When I rebuilt engines, I always primed the lubrication system mechanically before I turned the crankshaft with the starter. 

    -Engine overhaul mechanic and general automotive mechanic since 1972

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  • 1 month ago

    The new oil filter is empty and until it fills up, the oil pressure is too low to prevent metal-to-metal friction. I suspect the connecting rod bearings were very worn out. It also could be valve lifters needed to pump up to close rocker lash and clatter. 

  • Anonymous
    1 month ago

    On some engines, the oil pump loses it's prime when the oil is drained out.......and with a new filter it can take a few seconds to fill it with oil.  As others said, it's preferable to fill the new filter with oil before installing.  And "in the old days" we would pull the coil wire so we could crank the engine for a while to start pumping the oil around before reconnect the coil and starting.

    btw:  most oil pumps are submerged in the oil . . . but old Buicks had exterior mounted oil pumps - great to replace the pump without dropping the pan - but every time you started the engine you had the oil starvation you are experiencing.  Not a problem when it only happens at an oil change - but Buicks developed early bearing problems......they finally moved the pumps into the pan like other makes.

  • 1 month ago

    Weak oil pump and worn bearings.

  • Anonymous
    1 month ago

    Ur not doing it right. U warm up the engine so oil is on the parts, change it quickly, fill the oil filter with oil, screw it in, and then start the engine again to get the new filter completely full of oil. 

    If you just change the oil slowly and dont add oil to the filter then everything will be cold and there will be no oil on the parts for like 5-10 seconds. 

    That's why I change mine quickly.

    I start it again when I'm done to get oil into all the lines and filter. There is some anti-drainback valve on modern oil filters to hold oil in the filter and oil line leading to the engine even when its off.

  • 1 month ago

    Because all of the oil drained out of the motor and when a lube boy puts a new after market oil filter back on during the oil change he doesn't fill the new oil filter and it takes a good 30 seconds for the oil pump to fill the oil filter and motor oil passages to pump up the oil pressure. This is why you need to wait at least 20 seconds when starting the motor after sitting over night. The bearings can get damaged if you don't let the oil pressure come up before taking off.

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